As semiconductor device manufacturers continue to produce smaller devices, the requirements for photomasks used in the fabrication of these devices continue to tighten. Photomasks, also known as reticles or masks, typically consist of substrates (e.g., high-purity quartz or fused silica) that have an absorber layer (e.g., chrome or molybdenum silicide) formed on the substrate. The absorber layer includes a pattern representing a circuit image that may be transferred onto semiconductor wafers in a lithography system. As feature sizes of semiconductor devices decrease, the corresponding circuit images on the photomask also become smaller and more complex. Consequently, the quality of the mask has become one of the most crucial elements in establishing a robust and reliable semiconductor fabrication process.
Today, semiconductor manufacturers are searching for techniques to extend the use of optical lithography for manufacturing high-density ICs with critical dimensions of less than 130 nm. As feature sizes decrease, however, resolution for imaging a minimum feature size on the wafer with a particular exposure wavelength is limited by the diffraction of the light. Therefore, a shorter exposure wavelength, e.g., less than 400 nm, is required to image finer features on the wafer. Wavelengths targeted for future generations of optical lithography include 248 nm (KrF laser wavelength), 193 nm (ArF laser wavelength), and 157 nm (F2 laser wavelength).
At wavelengths below 400 nm, flatness of the photomask blank and resulting photomask is a concern. Any change in the flatness may result in registration errors in a semiconductor manufacturing process. Since a large number of photomasks (e.g., up to 50 for advanced designs) may be used to create a single integrated circuit, registration errors on a single photomask should be kept to a minimum. Although tool accuracy may contribute to registration error, it has been demonstrated that the stress of the absorber layer may cause the substrate to warp and thus, create registration errors.
One technique for reducing the stress of the absorber layer includes annealing a photomask blank after the absorber layer has been formed on the substrate. This technique, however, has a number of disadvantages. First, the annealing is performed after the absorber layer is formed on the substrate, which adds a step and additional time to a photomask manufacturing process. Second, heat used during the annealing process modifies optical properties associated with the absorber layer, which is undesirable if a specific transmission and/or phase shift is desired.